Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
44
a
b
Figure 2.25 a Scanning electron micrograph of human red
blood cells (× 3300). Each cell contains about 250 million
haemoglobin molecules. b Scanning electron micrograph of
red blood cells from a person with sickle cell anaemia. You
can see a normal cell and three or four sickled cells (× 3300).
BOX 2.4: Testing for the presence of proteins
Background information
All proteins have peptide bonds, containing
nitrogen atoms. These form a purple complex
with copper(II) ions and this forms the basis of the
biuret test.
The reagent used for this test is called biuret reagent.
You can use it as two separate solutions: a dilute solution
of potassium hydroxide or sodium hydroxide, and a
dilute solution of copper(II) sulfate. Alternatively, you can
use a ready-made biuret reagent that contains both the
copper(II) sulfate solution and the hydroxide ready mixed.
To stop the copper ions reacting with the hydroxide ions
and forming a precipitate, this ready-mixed reagent also
contains sodium potassium tartrate or sodium citrate.
Procedure
The biuret reagent is added to the solution to be tested.
No heating is required. A purple colour indicates that
protein is present. The colour develops slowly over
several minutes.
Collagen – a fibrous protein
Collagen is the most common protein found in animals,
making up 25% of the total protein in mammals. It is
an insoluble fibrous protein (Figure 2.26) found in skin
(leather is preserved collagen), tendons, cartilage, bones,
teeth and the walls of blood vessels. It is an important
structural protein, not only in humans but in almost all
animals, and is found in structures ranging from the body
walls of sea anemones to the egg cases of dogfish.
As shown in Figure 2.26b, a collagen molecule consists
of three polypeptide chains, each in the shape of a helix.
(This is not an α-helix – it is not as tightly wound.) These
three helical polypeptides are wound around each other,
forming a three-stranded ‘rope’ or ‘triple helix’. The three
strands are held together by hydrogen bonds and some
covalent bonds. Almost every third amino acid in each
polypeptide is glycine, the smallest amino acid. Glycine
is found on the insides of the strands and its small size
allows the three strands to lie close together and so form a
tight coil. Any other amino acid would be too large.
Each complete, three-stranded molecule of collagen
interacts with other collagen molecules running parallel
to it. Covalent bonds form between the R groups of amino
acids lying next to each other. These cross-links hold
many collagen molecules side by side, forming fibrils.
The ends of the parallel molecules are staggered; if they
were not, there would be a weak spot running right across
the collagen fibril. Finally, many fibrils lie alongside each
other, forming strong bundles called fibres.
The advantage of collagen is that it is flexible but it has
tremendous tensile strength, meaning it can withstand
large pulling forces without stretching or breaking. The
human Achilles tendon, which is almost pure collagen
fibres, can withstand a pulling force of 300 N per mm 2 of
cross-sectional area, about one-quarter the tensile strength
of mild steel.
Collagen fibres line up according to the forces they
must withstand. In tendons they line up in parallel
bundles along the length of the tendon, the direction of
tension. In skin, they may form layers, with the fibres
running in different directions in the different layers,
like cellulose in cell walls. In this way, they resist tensile
(pulling) forces from many directions.